Green scene:

Quiet down!

by Rebekah Bovenmyer

Have you ever driven along a highway and had to crank the radio just to hear over the sound of your tires on the pavement? When air gets trapped between the tires and the pavement, it creates a high-pitch whine. It’s loudest when you’re driving more than 45 miles per hour.

Pavement noise can be annoying for drivers, but noise pollution is a problem for people living in urban areas. Highway designers have to add walls to their project plans to lower the noise level for the neighbors.

Find the right tine

Concrete streets and roads often have little lines cut into them, called tines, before the concrete dries on a wet pavement. Tines cut horizontally (transversely) across the concrete improve cars’ friction and skid resistance but also make some of the noisiest pavements.

Transportation engineers don’t want to lose the safety benefits of tines, so they look for different ways of tining that keep the friction but don’t make as much noise.

Instead of cutting transverse tines, they are beginning to use longitudinal tines. These go up and down the concrete road in the direction the car is moving. So there’s still skid resistance, but it’s not as loud.

Diamonds are a pavement’s best friend

One of the most effective ways to make pavements quieter is through diamond grinding. Pavement is usually diamond ground when it’s uneven and cracked, causing that "bump ba bump" noise when you drive over it.

The process actually uses real industrial diamonds to cut off a thin layer of pavement. Diamonds are the hardest rock, so they can easily slice through concrete.

Thin circular diamond blades are stacked together to make a roller, called the cutting head. The cutting head is run over the concrete pavement to grind off the top, uneven layer. The thin blades make the pavement longitudinally textured and even.

A cutting head can cost between $60,000-80,000, so diamond grinding is too expensive to do on every road.

“Holey” moly

A relatively new concrete doesn’t need tines or texturing. It’s called pervious concrete, and it has friction and noise control built right in. Pervious concrete isn’t completely solid. It’s made of larger rocks without as much sand or tiny fill material. This leaves little holes and gaps in between the rocks.

This porous concrete makes for a really quiet ride. All the spaces release air in between the pavement and the tire so there’s no high-pitched whine. The gaps also create enough friction and skid resistance for the tires without needing to add tines for safety.

Pervious concretes in the U.S. are primarily used in parking lots or on roads with little traffic. (See "On (not completely) solid ground" for more.) But lately, they’ve started to be used on highways. About 4 to 6 inches of pervious concrete is placed on top of the current concrete highway.

Because of the gaps in the concrete, rainwater runs into it. Pavers create a slope from the center of the road to drain it out onto the shoulder. So pervious concrete stays drier than regular concrete and is a whole lot quieter.